Please do "About this Mac" from your Apple menu. Reprot what is says for the Mac OS version AND the processor info. If it reports your processor as a "PowerPC like this:
you cannot upgrade higher than OS 10.5.8 "Leopard." If it says you have an Intel processor like this:
you can upgrade to at least 10.6.8 "Snow Leopard" and maybe farther, depending on some extra info we'll need to get from you.